[0001] The present invention relates to a method of pickling of steel in an acidic aqueous
pickling solution containing Fe
3+ and Fe
2+. The pickling capability of the bath is maintained by continuous supply of hydrogen
peroxide.
[0002] At manufacturing of steel, particularly stainless steel, an oxide layer forms at
the surface during the annealing, and this layer must be removed. This is normally
done by pickling which means that the steel is treated in an acidic oxidizing pickling
bath to affect some dissolution of metal under the oxide layer which then comes loose.
For a long time, pickling of stainless steel has often been performed in pickling
baths based on nitric acid as an oxidizing agent which, however, has involved emissions
of nitrous fumes and nitrates that are detrimental to the environment.
[0003] US patent 4938838 discloses addition of hydrogen peroxide for oxidizing nitrite to
nitrate in nitric acid based pickling baths. The emissions of nitrous fumes are significantly
reduced but are not totally eliminated, and the emissions of nitrates are not reduced
at all.
[0004] Pickling without nitric acid is disclosed in the US patents 5154774 and 5354383,
EP-A-582121, and in GB-A-2000196. These processes are based on the fact that Fe
3+ in the pickling bath acts as an oxidizing agent and is reduced to Fe
2+ at the same time as metallic iron in the steel is oxidized to Fe
2+. In order to maintain the oxidation potential in the pickling bath hydrogen peroxide
is added to reoxidize Fe
2+ to Fe
3+. An disadvantage of these processes is that the cost for hydrogen peroxide is rather
high since a great deal of it does not just react with Fe
2+ but also with other metal ions in the pickling bath, such as Fe
3+, and is then consumed to no use. It is also hard to achieve a sufficiently high pickling
rate.
[0005] JP-A-50-133125 (Chemical Abstracts 84:39369) relates to pickling of steel with a
ferrisulfate based solution. The pickling solution is regenerated batchwise by addition
of hydrogen peroxide in a separate tank.
[0006] US patent 3962005 discloses a method of etching a shadow mask comprising a steel
substrate with a ferric sulfate solution to make apertures in said substrate The etchant
is held in a reservoir and pumped through a pump to a nozzle for spraying onto the
mask. The spent solution is then collected again in the reservoir. A portion of the
etchant from the pump is supplied with hydrogen peroxide for regeneration.
[0007] The invention concerns a method of pickling of steel, preferably stainless steel,
with an acidic aqueous pickling solution containing Fe
3+ and Fe
2+. The steel is contacted with pickling solution by immersing it into a bath of pickling
solution that continuously is brought to circulate through a conduit into which hydrogen
peroxide is supplied to oxidize Fe
2+ to Fe
3+, wherein hydrogen peroxide is supplied in such an amount that the pickling solution
in said bath has a content of Fe
2+ of at least 0.2 grams/litre and is substantially free from hydrogen peroxide.
[0008] The invention further concerns a method of pickling of steel in an acidic aqueous
pickling solution containing Fe
3+ and Fe
2+, comprising the steps of contacting the steel with pickling solution by spraying
pickling solution onto the steel and then collecting said pickling solution into a
tank; bringing pickling solution to continuously circulate through a circulation conduit
by transferring pickling solution from said tank to said circulation conduit; supplying
hydrogen peroxide into said circulation conduit to oxidize Fe
2+ in the pickling solution therein to Fe
3+ and after completed oxidation spraying said pickling solution from the conduit onto
the steel; wherein hydrogen peroxide is supplied in such an amount that the pickling
solution sprayed onto the steel has a content of Fe
2+ of at least 0.2 grams/litre and is substantially free from hydrogen peroxide.
[0009] The object of the present invention is to provide an efficient and environmental
friendly process for pickling of steel with low consumption of hydrogen peroxide.
[0010] It has surprisingly been found that the consumption of hydrogen peroxide is significantly
lower if it, instead of being supplied directly to a bath, is fed into a special circulation
conduit. It is assumed that the reaction between hydrogen peroxide and Fe
2+ is considerably faster than the corresponding undesired reactions with other metal
ions. By feeding the hydrogen peroxide in a circulation conduit, there is always Fe
2+ present to come in contact with the hydrogen peroxide, while it has been found that
there in a pickling bath, even with vigorous agitation, always may exist zones depleted
of Fe
2+. In order to minimize the consumption of hydrogen peroxide it is preferably supplied
in such an amount that the pickling solution the steel is contacted with is substantially
free from hydrogen peroxide.
[0011] The hydrogen peroxide is preferably supplied in such an amount that the content of
Fe
2+ in the pickling solution the steel is contacted with becomes from about 0.2 to about
35 grams/litre, particularly from about 1 to about 20 grams/litre, and preferably
so the content of Fe
3+ becomes from about 15 to about 80 grams/litre, particularly from about 25 to about
55 grams/litre. It is then preferred that the molar ratio Fe
2+:Fe
3+ becomes from about 0.01:1 to about 1:1, particularly from about 0.05:1 to about 0.25:1.
Preferably, from about 0.3 to about 0.5 kg H
2O
2 (calculated as 100%) is added per kg Fe
2+ to be oxidized in the circulating pickling solution. The total content of iron ions,
i.e. Fe
2+ and Fe
3+ in the pickling solution is suitable from about 15 to about 100 grams/litre, preferably
from about 35 to about 65 grams/litre. The above contents of Fe
2+ and Fe
3+ refer to the solution in the circulation conduit before it comes into contact with
the steel.
[0012] According to an advantageous embodiment, the supply of hydrogen peroxide is controlled
on the basis of the redox potential in the pickling solution. The redox potential
in the solution mainly depends on the ratio Fe
2+:Fe
3+, the acidity and the temperature. If the last two parameters are kept constant, the
redox potential is a measure of the ratio Fe
2+:Fe
3+. Suitably the pickling solution is initially prepared with selected acidity and Fe
2+:Fe
3+ ratio and the redox potential then measured can be used as a set value for the regulation.
Initially, as well as now and then during the pickling, the Fe
2+ content can be measured by permanganate titration while the total iron content and
the acidity can be measured with commercially available instruments, such as ScanaconTMSA-20
which is based on measurement of acid concentration on ion-selective electrodes for
fluoride and hydrogen ions and measurement of the total iron content based on density
corrected for the concentration of acids and other metals. Preferably the redox potential
is measured in the circulation conduit after hydrogen peroxide has been supplied and
has reacted with Fe
2+. Depending on the design of the plant and the circulation rate of the pickling solution,
the redox potential may also be measured in the bath or just before the hydrogen peroxide
supply, preferably in combination with measurement also after the hydrogen peroxide
supply. Preferably a partial flow of the circulating pickling solution is divided
off for potential measurements, while measurements of acidity and iron content may
be performed on samples taken out manually. Preferably, the redox potential is maintained
from about 200 to about 600 mV, most preferably from about 300 to about 500 mV, measured
between platinum and a silver/silver chloride electrode.
[0013] Suitably the pickling solution is brought to circulate with help from a pump, wherein
the hydrogen peroxide preferably is supplied at the suction side of the pump which
results in a very effective mixing. Suitably the pickling solution is circulated with
a flow sufficient for maintaining a correct composition and redox potential in the
entire volume, which in most cases means that it is circulated with a space velocity
from about 0.5 to about 50 hours
-1, preferably from about 5 to about 15 hours
-1.
[0014] In one embodiment the steel is contacted with the pickling solution by being immersed
in a bath, which may be performed continuously by transporting a band or the like
through the bath, or batchwise by dipping objects such as wire coils or pipes in the
bath and optionally vibrating the objects simultaneously. Objects such as wire coils
may, for example, also be immersed into the bath a one end of the tub, be conveyed
to the other end of the tub, and finally be lifted up again. The pickling solution
in the bath is continuously circulating through a conduit into which hydrogen peroxide
is supplied and rapidly comes in contact with Fe
2+ so the solution has a suitable redox potential and suitable contents of Fe
2+ and Fe
3+ when it returns to the bath. If the hydrogen peroxide instead would have been added
directly to the bath, a great deal of it might go to zones depleted of Fe
2+ and then being lost in side reactions. The steel can also be immersed in two or more
baths after each others, preferably with individual circulation conduits and means
for feeding the hydrogen peroxide, in which baths the pickling solution may have substantially
the same or different compositions. It is also possible to perform one or more other
treatment steps between the baths, for example washing or mechanical treatment such
as brushing.
[0015] In another embodiment the steel is contacted with the pickling solution by spraying
it onto the steel and then collecting it into a tank. Collected pickling solution
is continuously circulating through a circulation conduit by transferred it from the
tank to the circulation conduit, into which hydrogen peroxide is supplied and rapidly
comes in contact with Fe
2+. After completed oxidation from Fe
2+ to Fe
3+ the pickling solution is sprayed onto the steel. If the hydrogen peroxide instead
would have been added directly to the tank a great deal of it would have been lost
in side reactions since there always exist zones with low or non-existent concentrations
of Fe
2+. Also in this embodiment the pickling may be performed continuously or batchwise
in one, two or several steps in sequence, optionally with intermediate treatment steps.
[0016] It is also possible first to spray pickling solution onto the steel and then immerse
the steel in a bath into which the sprayed pickling solution is collected.
[0017] The pickling solution suitably contains hydrofluoric acid, preferably from about
0.2 to about 5 mols/litre, measured as free fluoride, most preferably from about 1.5
to about 3.5 mols/litre. The hydrofluoric acid facilitates the pickling by complexing
iron.
[0018] In order to reach sufficiently high acidity, the pickling solution preferably contains
sulfuric acid, suitably from about 0.2 to about 5 mols/litre, preferably from about
1 to about 3 mols/litre.
[0019] Although normally not necessary, hydrogen peroxide with extra addition of stabilizers
may be used, for example containing from about 0.5 to about 30 grams stabilizers per
litre 35% hydrogen peroxide. Useful stabilizers comprises non-ionic surfactants such
as ethoxylated alcohols, for example C
10-14-alcohol connected with 7 ethylene oxide and 1 propylene oxide.
[0020] Suitably the pickling solution is substantially free from nitric acid, problems with
emissions of nitrous fumes or nitrates thus being avoided.
[0021] Suitably a temperature is maintained from about 30 to about 80°C, preferably from
about 35 to about 60°C.
[0022] In order to avoid accumulation and possible precipitations, metals such as iron are
preferably removed continuously from the pickling solution. This may, for example,
be performed with acid retardation in commercially available equipment such as Scanacon
™SAR 1100.
[0023] According to the invention, it has been found possible to combine high pickling rate
with low hydrogen peroxide consumption. Further, it is not necessary to blow air or
oxygen through the pickling solution as disclosed in the earlier mentioned US patents
5154774 and 5354383 since the circulation conduit contributes both to effective mixing
of the pickling solution and to efficient utilization of the hydrogen peroxide for
oxidation of Fe
2+.
[0024] The invention is now to be described in connection with the appended drawings, of
which the Figures 1 and 2 schematically show two different embodiments.
[0025] Figure 1 shows a tub 1 with a bath of pickling solution containing Fe
3+, Fe
2+, hydrofluoric acid, sulfuric acid and water, through which a running strip 2 of stainless
steel is conducted continuously. The pickling solution is brought to circulate through
a special conduit 4 with help from a pump 3. Hydrogen peroxide is supplied to the
conduit 4 on the suction side of the pump 3 from a storage tank 6 with help from a
feed pump 5. A partial flow from the circulation conduit 4 is led through an apparatus
7 for measurement of the redox potential and regulation of the feed pump 5 for hydrogen
peroxide. It is possible also to measure the redox potential in the tub 1 or before
the feed pump 5 and let the measured value control the set value for the redox potential
to be maintained at the end of the circulation conduit 4. Normally also hydrofluoric
acid and sulfuric acid are supplied continuously in order to compensate for losses
during the pickling.
[0026] Figure 2 shows an embodiment in which a steel strip 2 is pickled without being immersed
into the tub 1, instead pickling solution is sprayed onto the upper- and undersides
of the strip through nozzles 8 and is collected into the tub 1. In other aspects the
plant works as the one in Figure 1. Thus, pickling solution is pumped around in a
conduit 4 and is supplied with hydrogen peroxide at the suction side of the pump from
a storage tank 6 with a feed pump 5 which is controlled with redox measurement in
the apparatus 7. Although not shown in the figure, it is also possible to convey the
steel strip vertically and spray the pickling solution on the sides.
[0027] The invention is also illustrated in the following examples. In the absence of other
specification, all percentages refer to % by weight. All redox potentials are measured
between platinum and a silver/silver chloride electrode.
[0028] EXAMPLE 1: Not neolytic pretreated plates of stainless steel 17-11-2 Ti with a thickness
of 1.5 mm were pickled in a 20 litres bath consisting of an aqueous solution of 2.0
mols/litre H
2SO
4, 3.3 mols/litre HF, 10-11 grams/litre Fe
2+ and 69-70 grams/litre Fe
3+ for 7 minutes at a temperature of 60°C and a redox potential of 380 mV. In experiment
I the pickling solution was pumped around through a conduit so the space velocity
was about 40 hours
-1. 35% hydrogen peroxide solution was fed in this conduit. In experiment II the pickling
tub was provided with an agitator rotating with 60 r/min and 35% hydrogen peroxide
solution was fed directly into the tub. The results appear from the table below in
which the hydrogen peroxide consumption refer to 35% solution:
The results show that the hydrogen peroxide consumption was decreased and the pickling
rate increased when the hydrogen peroxide was fed in a circulation conduit.
[0029] EXAMPLE 2: In a full scale plant a 1270 mm wide and 0.6 mm thick band of neolytic
pretreated stainless steel 17-12-2,5 L was pickled continuously with a speed of 35
meters/minute in two 12 m
3 tubs placed in sequence. In each one of the tubs the pickling solution was pumped
around in a circulation conduit into which 35% hydrogen peroxide was fed, wherein
the space velocity of the pickling solution in each tub was about 3 hours
-1. The total hydrogen peroxide consumption was about 30 ml 35% solution per m
2 pickled material. The first tub contained at steady state an aqueous solution of
2.69 mols/l HF, 1.82 mols/l H
2SO
4, 2.5 g/l Fe
2+ and 44.5 g/l Fe
3+, while the temperature was 60°C and the redox potential was 439 mV. The second tub
contained at steady state an aqueous solution of 2.58 mols/l HF, 1.74 mols/l H
2SO
4, 2.2 g/l Fe
2+ and 34.8 g/l Fe
3+, while the temperature was 61°C and the redox potential was 452 mV. The pickling
was approved by the regular controller of the plant.
[0030] EXAMPLE 3: In a full scale plant a 1250 mm wide and 2.0 mm thick band of neolytic
pretreated and grind brushed stainless steel 904 L was pickled continuously with a
speed of 10 meters/minute in two 12 m
3 tubs placed in sequence. In each one of the tubs the pickling solution was pumped
around in a circulation conduit into which 35% hydrogen peroxide was fed, wherein
the space velocity of the pickling solution in each tub was about 3 hours
-1. The total hydrogen peroxide consumption was about 30 ml 35% solution per m
2 pickled material. The first tub contained at steady state an aqueous solution of
3.16 mols/l HF, 1.8 mols/l H
2SO
4, 1.7 g/l Fe
2+ and 45.3 g/l Fe
3+, while the temperature was 61°C and the redox potential was 442 mV. The second tub
contained at steady state an aqueous solution of 3.15 mols/l HF, 1.7 mols/l H
2SO
4, 2.6 g/l Fe
2+ and 39.4 g/l Fe
3+, while the temperature was 62°C and the redox potential was 453 mV. The pickling
was approved by the regular controller of the plant.
1. A method of pickling of steel in an acidic aqueous pickling solution containing Fe3+ and Fe2+, comprising the steps of contacting the steel with pickling solution by immersing
it into a bath of pickling solution; continuously circulating the pickling solution
in said bath through a conduit; and supplying hydrogen peroxide into said conduit
to oxidize Fe2+ in the pickling solution to Fe3+; wherein hydrogen peroxide is supplied in such an amount that the pickling solution
in said bath has a content of Fe2+ of at least 0.2 grams/litre and is substantially free from hydrogen peroxide.
2. A method of pickling of steel in an acidic aqueous pickling solution containing Fe3+ and Fe2+, comprising the steps of contacting the steel with pickling solution by spraying
pickling solution onto the steel and then collecting said pickling solution into a
tank; bringing pickling solution to continuously circulate through a circulation conduit
by transferring pickling solution from said tank to said circulation conduit; supplying
hydrogen peroxide into said circulation conduit to oxidize Fe2+ in the pickling solution therein to Fe3+ and after completed oxidation spraying said pickling solution from the conduit onto
the steel; wherein hydrogen peroxide is supplied in such an amount that the pickling
solution sprayed onto the steel has a content of Fe2+ of at least 0.2 grams/litre and is substantially free from hydrogen peroxide.
3. A method as claimed in any one of the claims 1-2, wherein the pickling solution is
brought to circulate through the circulation conduit with a space velocity from about
0.5 to about 50 hours-1.
4. A method as claimed in any one of the claims 1-3, wherein the pickling solution is
brought to circulate with a pump and that the hydrogen peroxide is supplied at the
suction side of said pump.
5. A method as claimed in any one of the claims 1-4, wherein hydrogen peroxide is supplied
in such an amount that the weight ratio Fe2+:Fe3+ becomes from about 0.01:1 to about 1:1 in the pickling solution the steel is contacted
with.
6. A method as claimed in any one of the claims 1-5, wherein hydrogen peroxide is supplied
in such an amount that the content of Fe2+ becomes from about 0.2 to about 35 grams/litre in the pickling solution the steel
is contacted with.
7. A method as claimed in any one of the claims 1-6, wherein the pickling solution contains
hydrofluoric acid.
8. A method as claimed in any one of the claims 1-7, wherein the pickling solution contains
sulfuric acid.
9. A method as claimed in any one of the claims 1-8, wherein the pickling solution is
substantially free from nitric acid.
1. Verfahren zum Beizen von Stahl in einer wäßrig sauren Beizlösung, die Fe3+ und Fe2+ enthält, umfassend die Schritte des Inkontaktbringens des Stahls mit der Beizlösung
durch Eintauchen in ein Bad der Beizlösung; des kontinuierlichen Umwälzens der Beizlösung
in dem Bad durch eine Rohrleitung; und des Zuführens von Wasserstoffperoxid in die
Rohrleitung zur Oxidation von Fe2+ in der Beizlösung zu Fe3+; wobei Wasserstoffperoxid in einer solchen Menge zugeführt wird, daß die Beizlösung
in dem Bad einen Fe2+-Gehalt von mindestens 0,2 g/Liter aufweist und im wesentlichen frei von Wasserstoffperoxid
ist.
2. Verfahren zum Beizen von Stahl in einer wäßrig sauren Beizlösung, die Fe3+ und Fe2+ enthält, umfassend die Schritte des Inkontaktbringens des Stahls mit der Beizlösung
durch Sprühen der Beizlösung auf den Stahl und dann Auffangen der Beizlösung in einem
Tank; des Herbeiführens einer kontinuierlichen Umwälzung der Beizlösung durch eine
Umlaufleitung durch Überführung von Beizlösung aus dem Tank in die Umlaufleitung;
des Zuführens von Wasserstoffperoxid in die Umlaufleitung zur Oxidation von Fe2+ in der Beizlösung zu Fe3+, und Sprühens der Beizlösung nach beendeter Oxidation aus der Leitung auf den Stahl;
wobei Wasserstoffperoxid in einer solchen Menge zugeführt wird, daß die auf den Stahl
gesprühte Beizlösung einen Fe2+-Gehalt von mindestens 0,2 g/Liter aufweist und im wesentlichen frei von Wasserstoffperoxid
ist.
3. Verfahren nach einem der Ansprüche 1-2, wobei die Beizlösung durch die Umlaufleitung
mit einer Raumgeschwindigkeit von etwa 0,5 bis etwa 50 Stunden-1 in Umlauf gebracht
wird.
4. Verfahren nach einem der Ansprüche 1-3, wobei die Beizlösung mit einer Pumpe in Umlauf
gebracht wird und das Wasserstoffperoxid an der Saugseite der Pumpe zugeführt wird.
5. Verfahren nach einem der Ansprüche 1-4, wobei Wasserstoffperoxid in einer solchen
Menge zugeführt wird, daß das Gewichtsverhältnis von Fe2+:Fe3+ in der Beizlösung, mit der der Stahl in Kontakt gebracht wird, etwa 0,01:1 bis etwa
1:1 beträgt.
6. Verfahren nach einem der Ansprüche 1-5, wobei Wasserstoffperoxid in einer solchen
Menge zugeführt wird, daß der Fe2+-Gehalt in der Beizlösung, mit der der Stahl in Kontakt gebracht wird, etwa 0,2 bis
etwa 35 g/Liter beträgt.
7. Verfahren nach einem der Ansprüche 1-6, wobei die Beizlösung Fluorwasserstoffsäure
enthält.
8. Verfahren nach einem der Ansprüche 1-7, wobei die Beizlösung Schwefelsäure enthält.
9. Verfahren nach einem der Ansprüche 1-8, wobei die Beizlösung im wesentlichen frei
von Salpetersäure ist.
1. Procédé de décapage d'acier dans une solution de décapage aqueuse acide qui contient
Fe3+ et Fe2+, comprenant les étapes consistant à mettre l'acier en contact avec une solution de
décapage en l'immergeant dans un bain de solution de décapage ; à faire circuler en
continu la solution de décapage dans ledit bain à travers une conduite ; et à introduire
du peroxyde d'hydrogène dans ladite conduite pour oxyder Fe2+ dans la solution de décapage en Fe3+ ; dans lequel on introduit le peroxyde d'hydrogène en une quantité telle que la solution
de décapage dans ledit bain présente une teneur en Fe2+ d'au moins 0,2 g/l et est pratiquement exempte de peroxyde d'hydrogène.
2. Procédé de décapage d'acier dans une solution de décapage aqueuse acide qui contient
Fe3+ et Fe2+, comprenant les étapes consistant à mettre l'acier en contact avec une solution de
décapage en pulvérisant la solution de décapage sur l'acier, puis à recueillir ladite
solution de décapage dans une cuve ; à faire circuler la solution de décapage en continu
à travers une conduite de circulation en transférant la solution de décapage depuis
ladite cuve vers ladite conduite de circulation ; à introduire du peroxyde d'hydrogène
dans ladite conduite de circulation pour oxyder Fe2+ dans la solution de décapage en Fe3+ et après que l'oxydation soit terminée, à pulvériser ladite solution de décapage
depuis la conduite jusque sur l'acier ; dans lequel on introduit le peroxyde d'hydrogène
en une quantité telle que la solution de décapage pulvérisée sur l'acier présente
une teneur en Fe2+ d'au moins 0,2 g/l et est pratiquement exempte de peroxyde d'hydrogène.
3. Procédé selon l'une quelconque des revendications 1 et 2, dans lequel on fait circuler
la solution de décapage à travers la conduite de circulation avec une vitesse horaire
d'environ 0,5 à environ 50 h-1.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel on fait circuler
la solution de décapage avec une pompe et le peroxyde d'hydrogène est introduit du
côté de l'aspiration de ladite pompe.
5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel on introduit
le peroxyde d'hydrogène en une quantité telle que le rapport en poids Fe2+ : Fe3+ passe d'environ 0,01:1 à environ 1:1 dans la solution de décapage avec laquelle l'acier
est en contact.
6. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel on introduit
le peroxyde d'hydrogène en une quantité telle que la teneur en Fe2+ passe d'environ 0,2 à environ 35 g/l dans la solution de décapage avec laquelle l'acier
est en contact.
7. Procédé selon l'une quelconque des revendications 1 à 6, dans lequel la solution de
décapage contient de l'acide fluorhydrique.
8. Procédé selon l'une quelconque des revendications 1 à 7, dans lequel la solution de
décapage contient de l'acide sulfurique.
9. Procédé selon l'une quelconque des revendications 1 à 8, dans lequel la solution de
décapage est pratiquement exempte d'acide nitrique.